Optics: measuring and testing – Shape or surface configuration
Reexamination Certificate
1999-12-06
2002-05-14
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
Shape or surface configuration
C356S602000, C250S559220
Reexamination Certificate
active
06388754
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a shape measuring system and method wherein an intensity-modulated light is emitted toward an object and the distance up to the object is measured on the basis of a phase difference between the light reflected from the object and the emitted light. Particularly, the present invention is concerned with a shape measuring system of reduced size and cost and a shape measuring method both capable of measuring the distance up to an object accurately and independently of external conditions such as a change in reflectance of the object surface.
2. Description of the Prior Art
As methods for measuring a three-dimensional shape there have been proposed two methods which are a passive method and an active method. The passive method measures the shape of an object without radiating energy to the object, while the active method measures the shape of an object by radiating a certain energy to the object and detecting its reflection.
As an example of the passive method there is known a stereo method which measures distances at plural points up to an object. According to the stereo method, two cameras are disposed at a certain interval and the distance up to an object is determined by triangulation on the basis of a parallax of two images obtained. This method is advantageous in that a remote distance can be measured if only images can be picked up, but involves a serious problem that it is impossible to make a three-dimensional measurement for the whole of a smooth surface free of any pattern. In addition, since it is basically impossible to align the optical axes of two cameras with each other, there has been a drawback that there occurs an area (occlusion) incapable of measuring distance.
As to the active method, a light-stripe method is mentioned as an example of the method which measures distances at plural points up to an object. According to this light-stripe method, a slit light is radiated to an object at a certain angle and the distance up to the object is determined by triangulation on the basis of an image picked up at an angle different from the said radiation angle. This method is characteristic in that a relatively simple construction suffices, but it is required that the slit light be scanned in a very small angular unit, resulting in that the measurement time is long because an image is picked up at every angular unit. For solving this problem there has been proposed a structured light method which utilizes the light-stripe method. According to the structured light method, a pattern of a projection light is coded instead of radiating the slit light many times and it is thereby intended to measure a distance with a reduced number of projections. However, given that the number of samples in the horizontal direction is n, it is necessary that image pick-up be conducted log2 n times (nine times if n=512 points), thus giving rise to the problem that the measurement time becomes longer. Additionally, since it is basically impossible to align the optical axis of a projector and that of an image pick-up device with each other, a drawback has been encountered that there occurs an area (occlusion) incapable of measuring a distance.
In connection with the active method, as an example of the method capable of measuring distances at plural points by one image pick-up, there is known a phase distribution measuring method wherein an intensity-modulated light is radiated to an object and a phase distribution of reflected light is measured.
As examples of conventional phase distribution measuring methods are mentioned methods disclosed in Literature 1, “An article described on pages 126 to 134 of SPIE Vol. 2588 (1995) (A new active 3D-Vision system based on rf-modulation interferometry light),” Japanese Patent No. 2690673, and SPIE Vol. 2748 (1996), pp. 47-59, “The Emerging Versatility of a Scannless Range Imager.”
FIG. 10
illustrates the conventional shape measuring system described in Literature 1. This shape measuring system, indicated at
100
, comprises a modulation/demodulation signal generator
104
which applies an intensity modulation to light emitted from a light source
101
A through a condenser lens
102
to a plane modulator
103
using such a crystal as Pockels cell, a projection lens
106
which projects an intensity-modulated light
105
a
onto an object
6
planewise, the modulation/demodulation signal generator
104
which applies an intensity demodulation to a reflected light
105
b
incident on a plane demodulator
108
through a focusing lens
107
after being reflected by the object
6
, the plane demodulator
108
using such a crystal as Pockels cell, and a CCD camera
109
which picks up an intensity-demodulated light signal. According to this construction, light emitted from the light source
101
A is directed to the plane modulator
103
by the condenser lens
102
and is intensity-modulated in accordance with a signal produced from the modulation/demodulation signal generator
104
. Thereafter, the intensity-modulated signal
105
a
is projected planewise onto the object
6
by the projection lens
106
. The reflected light
105
b
from the object
6
is introduced into the plane demodulator
108
by the focusing lens
107
and is intensity demodulated in accordance with a signal provided from the modulation/demodulation signal generator
104
, then is focused on the CCD camera
109
. An intensity image picked up by the CCD camera
109
contains a phase information based on the distance up to the object
6
. By processing this intensity image with a computer
110
it is possible to obtain distance data of the object
6
at a single pick-up of image.
FIG. 11
illustrates the conventional shape measuring system disclosed in Japanese Patent No. 2690673. This shape measuring system is different in the following three points from the shape measuring system shown in FIG.
10
. The first point is the use of a semiconductor laser
101
B as a light source, the second point is that intensity modulation is performed directly by the semiconductor laser
101
B without the use of a modulator using such a crystal as Pockels cell, and the third point is that demodulation is performed by an image intensifier
111
without the use of a demodulator using such a crystal as Pockels cell. Light which has been subjected to intensity modulation in accordance with a signal provided from the modulation/demodulation signal generator
104
is radiated from the semiconductor laser
101
B and is then projected planewise onto the object
6
by the projection lens
106
. Reflected light
105
b
from the object
6
is focused on the image intensifier
111
by the focusing lens
107
. The signal from the modulation/demodulation signal generator
104
is converted to a high-voltage signal by a high-voltage drive circuit
112
, which signal is inputted to a gain controller terminal of the image intensifier
111
. Thus, the intensity-demodulated reflected light is picked up by the CCD camera
109
. A intensity image obtained in the CCD camera
109
contains phase information based on the distance up to the object
6
. By processing this intensity image with the computer
110
it is possible to obtain distance data of the object
6
at a single pick-up of image.
However, the shape measuring system shown in
FIG. 10
is disadvantageous in that the cost thereof is very high because a modulator/demodulator using such a crystal as Pockels cell is used for each of the plane modulator
103
and the plane demodulator
108
. Moreover, since the modulator/demodulator using such a crystal is of a small aperture which is several millimeters or so, it is necessary that the light emitted from the light source
101
A and the light reflected by the object
6
be condensed in conformity with the said aperture by the condenser lens
107
, with consequent increase in system size.
The shape measuring system shown in
FIG. 11
is also disadvantageous in that it is very expensive because the image intensifier
111
is used. More
Nishikawa Osamu
Tokai Kiwame
Yamaguchi Yoshinori
Fuji 'Xerox Co., Ltd.
Pham Hoa Q.
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